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Chapter 27 Fluid, Electrolyte, and Acid-Base Balance: An Overview ▪Extracellular fluid- ECF▪Intracellular fluid- ICF▪Normal volume and composition of these fluids is necessary for survival. ▪Stabilizing the volumes, solute concentrations, and pH of the ECF and the ICF involves three interrelated processes: 1. Fluid balance2. Electrolyte balance3. Acid-Base BalanceAn Introduction to Fluid and Electrolyte Balance ▪The ECF and the ICF- exchange between these two fluids occur across cell membranes by osmosis, diffusion, and carrier-mediated transport. ▫ECF is subdivided into interstitial fluid and plasma. ▫Fluid compartments▫The principle ions in the ECF are: ▫The ICF contains an abundance of: ▫Despite the differences in the concentration of specific substances, the ICF and ECF osmotic concentrations are identical. ▪Basic Concepts in the Regulation of Fluids and Electrolytes- four basic principles necessary for the understanding of fluid and electrolyte balance. 1) 2) 3) 4) ▪An Overview of the Primary Regulatory Hormones- major physiological adjustments affecting fluid balance and electrolyte balance are mediated by 3 hormones: ADH, aldosterone, natriuretic peptides (ANP and BNP). ▫ADH- -Increased release of ADH has two important effects: 1) 2) ▫Aldosterone-The higher the plasma concentration of aldosterone, the more efficiently the kidneys conserve Na+. -Aldosterone also increases the sensitivity of salt receptors on the tongue, increase interest in and consumption of salty foods. -Aldosterone is secreted in response to rising K+ or falling Na+ levels in the blood that reaches the adrenal cortex, or in response to the renin-angiotensin system. ▫Natriuretic Peptides (ANP and BNP): -Reduce thirst, and block the release of ADH and aldosterone, results in diuresis which lowers both blood pressure and plasma volume. ▪The Interplay Between Fluid Balance and Electrolyte Balance- must be seen as separate entities, even though they are very closely related. Fluid Balance ▪Water moves freely within the ECF compartment, allowing for important circulation among minor compartments. ▪Fluid Movement within the ECF ▫The relationship between the net hydrostatic pressure (NHP) and the blood colloid osmotic pressure (BCOP) determine the exchange between the plasma and the interstitial fluid. ▫This results in continuous filtration of fluid from the capillaries into the interstitial fluid. ▫Any factor that affects the NHP or BCOP will alter the distribution of fluid within the ECF. ▪Fluid Gains and Losses- common routes of fluid exchange with the environment ▫Water losses-Temperature rise accompanying a fever▫Water gains- ▪Fluid Shifts- water movement between the ECF and the ICF ▫Fluid shifts occur rapidly in response to changes in the osmotic concentration of the ECF and reach equilibrium within minutes to hours. ▫If the osmotic concentration of your ECF increases: ▫If the osmotic concentration of your ECF decreases: ▫Allocation of water loss-Dehydration (water depletion) -Hypernatremia- ▫Distribution of water gains-Hyperhydration-Hyponatremia results when: 1) 2) 3) Electrolyte Balance ▪Electrolyte balance is important because: ▫Total electrolyte concentrations directly affect water balance ▫The concentrations of individual electrolytes can affect cell functions ▪Na+ and K+ are important because they are major contributors to the osmotic concentrations of the ECF and ICF, respectively. They directly effect normal functioning of cells. -More than 90% of the osmotic concentration of the ECF results from the presence of sodium salts (sodium chloride and sodium bicarbonate) -Normal concentrations of Na+ in body fluids averages 140 mEq/L while inside cells, it is 10 mEq/L -Potassium concentrations within cells reach 160 mEq/L, while only around 3.8 to 5 mEq/L in body fluids. ▫Two general rules about sodium and potassium balance: 1) 2) ▪Sodium Balance- the total amount of sodium in the ECF represents a balance between two factors: 1) 2) ▫A change in sodium content of the ECF does not produce a change in the sodium concentration. -A corresponding gain or loss of water tends to keep the sodium concentration constant. ▫When sodium gains exceed sodium losses: ▫When sodium losses exceed sodium gains: ▫ECF volume changes are related to blood plasma volume, thus blood volume changes. Blood volume changes are monitored by looking at blood pressure. *see figure 27-5 and 27-6 to familiarize yourself with fluid volume regulation and sodium ion concentrations in body fluids.* ▪Potassium Balance- roughly 98% of the K+ content of the human body is in the ICF. ▫The K+ concentration in the EFC is a balance between K+ absorbed at the digestive epithelium and the loss at the kidney. ▫Urinary K+ losses are usually limited to the amount gained by absorption. (50-150 mEq/L per day) ▫The K+ concentration in the ECF is controlled by adjustments in the rate of active secretion along the DCT and collecting system, rate varies in response to: 1) 2) 3) ▫Hypokalemia▫Hyperkalemia▪Other Electrolytes ▫Calcium balance- calcium is the most abundant mineral in the body, 99% deposited in the bones. ▫Magnesium balance- more Mg2+ is found in the ICF, about 26 mEq/L, 1.5-2.5 mEq/L is found in the ECF. ▫Phosphate balance- phosphate ions are bound up in the mineral salts of the skeleton, and used for formation of high energy compounds, activation of enzymes, and synthesis of nucleic acids. ▫Chloride balance- most abundant anions in the ECF. Acid-Base Balance ▪The Importance of pH Control ▫Any deviation outside the normal range (7.35-7.45) is extremely dangerous, because changes in H+ concentrations disrupt the stability of cell membranes, alter the structure of proteins, and change the activities of important enzymes. ▫The nervous and cardiovascular systems are particularly sensitive to fluctuations in pH, for example, severe acidosis (under 7.0) can be deadly because: 1) 2) 3) ▫Acidosis is more common, due to the fact that acids are produced by normal cellular activities. ▪Types of Acids in the Body- there are three categories of acids in the body; volatile acids, fixed acids, and organic acids. ▫Volatile acids- ▫Fixed acids▫Organic acids▪Mechanisms of pH Control ▫To maintain acid-base balance, your body must balance gains and losses of hydrogen ions. ▫Buffer system▫Phosphate buffer system▫Protein buffer system- depends on the ability of amino acids to respond to changes in pH by accepting or releasing H+ ▫Hemoglobin buffer system- hemoglobin molecules buffer the H+ concentrations as carbonic acid dissociates. ▫Carbonic Acid-Bicarbonate Buffer System- -The primary role of the carbonic acid-bicarbonate buffer system is to prevent changes in pH caused by organic acids and fixed acids in the ECF. There are three important limitations: 1) 2) 3) ▪Maintenance of Acid-Base Balance- buffer systems cannot work for an extended period of time, they temporarily tie up H+, rendering them harmless. ▫The maintenance of acid-base balance includes balances H+ gains and losses, involves coordinating the buffer systems with the renal and respiratory systems. These mechanisms support buffer systems by: 1) 2) 3) ▫Respiratory compensation- change in the respiratory rate that helps stabilize the pH of the ECF. *review figure 23-26 for a reminder of this mechanism* ▫Renal compensation- change in the renal rates of H+ and HCO3secretion or reabsorption in response to changes in plasma pH. *see figure 27-10 for illustrations of these buffer systems* -When body fluid pH drops (acidosis), the kidney’s respond by: -When body fluid pH increases (alkalosis), the kidney’s respond by: Disturbances of Acid-Base Balance ▪Serious and prolonged disturbances of acid-base balance can result from any factor that affects one of the principal regulatory mechanisms: ▪Acute phase▪Compensated phase▪Respiratory Acidosis▫The primary symptom is low plasma pH due to hypercapnia (elevated plasma pCO2) ▫Acute respiratory acidosis▫Chronic respiratory acidosis-CNS injuries, desensitized respiratory centers, emphysema, congestive heart failure, pneumonia are causes of chronic respiratory acidosis. -Kidney’s will increase the rate of H+ secretion into tubular fluid, but cannot alone return the pH to normal until the underlying respiratory or circulatory problems are corrected. -Artificial respiration, mechanical ventilator, or induced bronchodilation can be used to increase pulmonary exchange. ▪Respiratory Alkalosis- relatively uncommon, develops when respiratory activity lowers plasma pCO2 levels to below normal levels, hypocapnia. ▫Hyperventilation can cause temporary hypocapnia. ▫If continued, individual will become unconscious and the breathing rate returns to normal. ▫Other problems with respiratory alkalosis are rare and involve: ▪Metabolic Acidosis- second most common type of acid-base imbalance, it has three major causes: 1) 2) 3) ▫Respiratory and metabolic acidosis are typically linked, because oxygenstarved tissues generate large quantities of lactic acid and because sustained hypoventilation leads to decreased PO2. ▪Metabolic Alkalosis- occurs when HCO3- concentrations become elevated, taking H+ out of solution and generating H2CO3 ▪The Detection of Acidosis and Alkalosis *use figure 27-18 and table 27-4 to help you identify acid-base disorders*